Rotary and oscillating sprinkler systems are widely used to irrigate lawns and landscaping in both commercial and residential environments. The most effective and reliable sprinkler systems include a series or network of pop-up sprinkler heads connected to a fluid source via irrigation pipes installed underground around the area to be maintained, an example of which is illustrated in FIG. 1. Each pop-up type rotary sprinkler head 100 generally includes a riser assembly 102 which is slidably mounted in the sprinkler head housing 104 between a fully retracted position in which the riser assembly 102 is entirely encased within the housing 104 when no fluid is flowing through the sprinkler head 100, and a fully extended position in which substantially the entire length of the riser assembly 102 extends out of the housing 104 when fluid is flowing through the sprinkler head 100 during operation of the irrigation system. A nozzle assembly 108 is rotatably attached at the top of riser assembly 102, and includes at least one nozzle 110 through which irrigation fluid is distributed out of the sprinkler head 100.
The sprinkler head housing is typically installed just beneath the ground surface 106 so that when no fluid is flowing into the sprinkler head, the riser assembly is also substantially below the ground surface. When irrigation fluid flows through the sprinkler head, the force of fluid pushes the riser assembly out of the housing until the riser assembly is fully extended to be appropriately positioned above the ground surface to deliver irrigation fluid.
FIG. 2 shows an example of a riser assembly as disclosed in U.S. Patent Application Publication No. 2002/0074432, and which includes a turbine assembly 122 having a rotor 112 and a turbine inlet 114, a gear assembly 116, and a turbine shaft 118. During operation, fluid flowing into the riser assembly enters the turbine inlet 114 and causes the turbine wheel 112 to rotate. Rotor 112 is attached to turbine shaft 118, which drives the gears in gear assembly 116.
Nozzle assembly 108 is rotatably connected to the riser assembly 102 by an output shaft 120, which also defines the flow path of fluid from the riser assembly 102 into the nozzle assembly 108. As such, irrigation fluid flows upwardly through the riser assembly 102 and is channeled into output shaft 120 and out through nozzle 110. In the riser assembly 102, output shaft 120 is driven by the output of gear assembly 116, whereby rotation of the output shaft 120 is thereby controlled by the movement of the gears in the gear assembly 116. The gears may be configured to rotate the output shaft 120 continuously or in an oscillating manner through a predetermined arc, as disclosed, for example, in U.S. Pat. No. RE 35,037 to Kah, Jr. and U.S. Patent Application Publication No. 2002/0074432 to Kah, Jr. et al., the disclosures of which are both incorporated herein by reference.
In climates which experience freezing temperatures during the year, irrigation systems such as those described above must be drained or blown-out with air after seasonal use to clear any water out of the system to prevent freezing damage. In many cases, the simplest installation provides only for allowing the irrigation system pipes and sprinkler heads to be cleared of water by blowing out compressed air through the system. This can be very damaging to the turbines, which normally rotate at a much slower speed when driven by water. Air is an expandable fluid and is relatively light compared to water, which is a relatively incompressible fluid and does not generate the rotational velocities produced when air is expanded in the turbine assembly onto the rotor blades.
Unless care is taken to limit the system air, blow-out time and pressures, the high turbine shaft velocities resulting from blowing compressed air through the sprinkler system can heat the shaft and cause it to seize to the plastic housing material. Once this occurs, the rotor is prevented from turning any further and is rendered unusable in the future. This has proved to be one of the major causes for premature failure of gear driven sprinklers in colder climates, where sprinklers are used for only part of the year and would therefore be expected to last much longer than in warmer climates, where they are run year round. Accordingly, the longevity of gear driven sprinkler systems in colder climates would be greatly enhanced if such systems were equipped with means to prevent the turbine rotor from rotating at excessively high velocities when driven with compressed air.
At least one device is known for preventing excessive rotational speed in turbine-driven sprinklers. One such device is disclosed in U.S. Patent Application Publication No. 2002/0162901 to Hunter et al., in which a brake force is applied to the rotor in a turbine assembly in a rotary sprinkler head when compressed air is flushed through the sprinkler system. To achieve this result, the turbine assembly includes a float mechanism which may be seated on the turbine rotor or blocks the flow path to the rotor when air is flowing through the sprinkler head, and is lifted off the rotor or removed from obstructing the flow path when water is delivered therethrough. The default position of the float mechanism is in the position to hinder rotation of the turbine rotor, but its buouyancy in water causes the float mechanism to be moved in the direction of flow so as to enable the turbine to rotate freely when water flows through the sprinkler head.
Even with water flowing through the sprinkler system, however, the sprinkler heads may wear out faster with continued operation at high fluid output rates than at lower output rates. In particular, certain types of rotary irrigation sprinkler systems provide the capability to adjust the output rates and/or change between several different nozzles for applying a selected flow rate and/or distribution profile of the irrigation fluid. Changes in the output flow rate caused by changing the nozzles also affect the flow rate driving the turbine rotor which rotates the sprinkler head. This is generally the case with most known rotary sprinklers, including the system disclosed in Hunter and discussed above. When the irrigation fluid flowing through the sprinkler system disclosed in Hunter is water, the rate of rotation of the turbine assembly is directly determined by the flow rate of water through the system, and would therefore vary through the entire operation range of the sprinkler system.
Because water is an incompressible fluid, as the selected output rate from the sprinkler increases, the faster the velocity of water passing through the turbine assembly. The faster the velocity of water entering the turbine assembly, the faster the rotor is driven by the water striking the rotor blades. Therefore, it would be advantageous to maintain the rotational velocity of the turbine rotor as constant as possible for as great a range of flow rates as possible for both air and water.